Underground drilling, such as gas, oil, or geothermal drilling, generally involves drilling a bore through a formation deep in the earth. Such bores are formed by connecting a drill bit to long sections of pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface to the bottom of the bore.
The drill bit is rotated so that the drill bit advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string at the surface. Piston-operated pumps on the surface pump high-pressure fluid, referred to as “drilling mud,” through an internal passage in the drill string and out through the drill bit. The drilling mud lubricates the drill bit, and flushes cuttings from the path of the drill bit. The drilling mud then flows to the surface through an annular passage fanned between the drill string and the surface of the bore.
The drilling environment, and especially hard rock drilling, can induce substantial vibration and shock into the drill string. Vibration also can be introduced by factors such as rotation of the drill bit, the motors used to rotate the drill string, pumping drilling mud, imbalance in the drill string, etc. Such vibration can result in premature failure of the various components of the drill string. Substantial vibration also can reduce the rate of penetration of the drill bit into the drilling surface, and in extreme cases can cause a loss of contact between the drill bit and the drilling surface.
Operators usually attempt to control drill string vibration by varying one or both of the following: the rotational speed of the drill string, and the down-hole force on the drill bit (commonly referred to as “weight-on-bit”). These actions often do not effectively reduce the vibrations. Reducing the weight-on-bit or the rotary speed of the drill bit usually reduces drilling efficiency. In particular, drill bits typically are designed for a predetermined rotary speed range and weight-on-bit. Operating the drill bit off of its design point can reduce the performance and the service life of the drill bit.
So-called “shock subs” are sometimes used to dampen drill string vibrations. Shock subs, however, typically are optimized for one particular set of drilling conditions. Operating the shock sub outside of these conditions can render the shock sub ineffective, and in some cases can actually increase drill string vibrations. Moreover, shock subs and isolators usually isolate the portions of the drill string up-hole of the shock sub or isolator from vibration, but can increase vibration in the down-hole portion of the drill string, including the drill bit.
An ongoing need therefore exists for a system and method that can dampen drill-string vibrations, and particularly vibration of the drill bit, throughout a range of operating conditions.